In recently years, the spread of the IEEE 802.11g standard, the IEEE 802.11a standard, or the like for high-speed wireless access systems using a 2.4 GHz band or a 5 GHz band has been remarkable. In these systems, a physical layer transmission speed of a maximum of 54 Mbps is realized using an orthogonal frequency division multiplexing (OFDM) modulation scheme, which is technology for stabilizing performance in a multipath fading environment (e.g., see Non-Patent Documents 1 and 4).
However, because the transmission speed referred to here is a transmission speed on the physical layer and the transmission efficiency in a medium access control (MAC) layer is actually about 50 to 70%, an upper-limit value of the actual throughput is about 30 Mbps and thus this performance is further degraded if the number of wireless communication stations intended to transmit information increases. On the other hand, in the field of wired local area networks (LANs), the provision of a high-speed link of 100 Mbps is widespread as a result of the spread of fiber to the home (FTTH) using optical fibers even in individual homes including a 100 Base-T interface of the Ethernet (registered trademark), and thus a further increase in a transmission speed is required even in the wireless LANs.
As technology for increasing the transmission speed, the extension of a channel bandwidth and a spatial multiplexing technology (multiple-input multiple-output (MIMO)) is introduced into the IEEE 802.11n standard. In addition, in the draft of the IEEE 802.11ac standard, the further extension of the channel bandwidth and a multiuser MIMO (MU-MIMO) transmission method to which a space division multiple access (SDMA) technology obtained by extending the spatial multiplexing technology is applied is being investigated (e.g., see Non-Patent Document 2). In addition, in the draft of the IEEE 802.11ac standard, a new concept called a group ID (GID) is prescribed. It is possible to simultaneously transmit data to all or part of wireless stations belonging to a group designated by a GID field of a frame by using the group ID.
Because the speed-up method by the extension of the channel bandwidth among the above-described speed-up technologies is easier to implement than the spatial multiplexing technology and the space division multiple access technology, it is a function implemented on many apparatuses. For example, the IEEE 802.11n standard has extended a channel bandwidth, which was fixed to 20 MHz in the IEEE 802.11a standard, to 40 MHz to thereby increase the speed. In addition, extension of a channel width to 80 MHz or 160 MHz is being investigated in the draft of the IEEE 802.11ac standard, which is currently being standardized in IEEE 802.11 Task Group ac (TGac). Here, for example, two adjacent 20 MHz channels are used when a width of 40 MHz is used and four adjacent 20 MHz channels are used when a width of 80 MHz is used.
In the wireless LAN system of the IEEE 802.11 standard, even when a wireless access point apparatus has a capability or function of performing transmission and reception in a wide band such as 40 MHz, 80 MHz, or 160 MHz as described above, a channel bandwidth available in actual transmission and reception is limited to a channel bandwidth supported by a wireless station apparatus (hereinafter referred to as a wireless station) associated with the wireless access point. That is, if it is impossible for the wireless station to transmit and receive a signal of a wide band such as 40 MHz, 80 MHz, or 160 MHz, the wireless access point has to perform transmission and reception of data using a channel bandwidth in a range that each wireless station can handle.
For example, the case in which the wireless access point can transmit and receive data using a band of 80 MHz conforming to the IEEE 802.11ac standard (draft) is considered. At this time, if the wireless station associated with the wireless access point can also use an 80 MHz mode conforming to the IEEE 802.11ac standard (draft), data transmission and data reception in the entire 80 MHz band between the wireless access point and the wireless station are possible. However, because a frequency available to the wireless station conforming to the IEEE 802.11a standard is 20 MHz, the above-described data transmission between the wireless access point and the wireless station is performed on one 20 MHz channel.
As described above, in the system conforming to the IEEE 802.11, it is difficult to sufficiently demonstrate the capability of the wireless access point if there is a difference in a channel bandwidth supported by the wireless station and the wireless access point. In addition, when the number of wireless stations of such a low function/capability increases, a frequency utilization efficiency and throughput performance of the entire system deteriorates.
Next, a method for wirelessly transmitting and receiving data in an IEEE 802.11 wireless LAN system will be described. In the wireless LAN system conforming to the IEEE 802.11, an access control procedure based on carrier sense multiple access with collision avoidance (CSMA/CA) is adopted and each wireless communication station (the wireless access point and the wireless station are collectively referred to as a wireless communication station) avoids a collision of a signal with that of another wireless communication station. A wireless communication station generating a transmission request first monitors a state of a wireless medium only in a predetermined sensing period (distributed inter-frame space (DIFS)). If there is no transmission signal by another wireless communication station during this period, the channel is regarded to be in an unused state (also referred to as an idle state) and the wireless communication station starts a random back-off procedure (a process of generating a random number within a predetermined range, determining a waiting time for collision avoidance control based on its value, and waiting for transmission for the time). The wireless communication station continues to monitor the wireless medium even during a random back-off period, and obtains an exclusive channel transmission right (transmission opportunity (TXOP)) over a predetermined period if there is no transmission signal by another wireless communication station even during the period. The wireless communication station obtaining the transmission right (TXOP) in this manner is referred to as a TXOP holder (hereinafter referred to as a transmission right acquiring wireless communication station). The wireless communication station becoming the transmission right acquiring wireless communication station can continuously transmit frames at very short time intervals referred to as short inter-frame spaces (SIFSs) without performing CSMA/CA within the TXOP period again.
In addition, there is “virtual carrier sense” as a method for solving a hidden terminal problem in wireless communication. Specifically, if Duration (continuous use period) information for providing the notification of a use time of wireless media is included when a wireless communication station receives a frame, the wireless communication station assumes that the media are used during a period corresponding to the Duration information (virtual carrier sense), sets the period as a transmission stop period (network allocation vector (NAV) period), and does not transmit a frame in the NAV period. Thereby, the exclusive use of the channel in the TXOP period is ensured.
When the wireless communication station receives the frame, the wireless communication station sets the NAV if necessary as described above and simultaneously records information (e.g., a MAC address) for identifying a transmission-source wireless communication station of the received frame, i.e., the transmission right acquiring wireless communication station, if the received frame is a frame which initiates the TXOP period (e.g., see Non-Patent Document 3). The stored information for identifying the transmission right acquiring wireless communication station is deleted when the TXOP period ends. It is to be noted that the frame initiating the TXOP period is not a special frame and it is a signal for reserving a channel over a fixed period by transmitting a control frame such as a request to send (RTS) frame.
When the wireless communication station receives a frame within the TXOP period again, the wireless communication station checks whether a transmission-source address of the received frame is the same as the MAC address stored as the information for identifying the transmission right acquiring wireless communication station. If they are the same, it is determined that the transmission-source wireless communication station of the received frame is the transmission right acquiring wireless communication station and a necessary reply frame is transmitted regardless of presence/absence of setting of the NAV within the wireless communication station itself. Thereby, the transmission right acquiring wireless communication station can transmit and receive data to and from a plurality of different wireless communication stations within the same TXOP period.
Hereinafter, an operation of transmitting and receiving a frame for transmitting and receiving data to be performed between wireless communication stations will be described with reference to FIGS. 72 to 74. FIG. 72 is a diagram illustrating a cell A of a wireless LAN constituted of one wireless access point AP1 and three wireless stations STA11 to STA13. It is assumed that the wireless access point AP1 and the wireless station STA13 conform to the IEEE 802.11ac standard and support three types of 20 MHz, 40 MHz, and 80 MHz as transmission/reception bandwidths. In addition, it is assumed that the wireless station STA11 conforms to the IEEE 802.11a standard, the wireless station STA12 conforms to the IEEE 802.11n standard, the wireless station STA11 supports a transmission/reception bandwidth of 20 MHz, and the wireless station STA12 supports transmission/reception bandwidths of 20 MHz and 40 MHz.
FIG. 73 is a time chart illustrating timings at which frames are transmitted when the transmission right acquiring wireless communication station transmits a plurality of frames addressed to other wireless communication stations within the TXOP. In FIG. 73, the horizontal axis represents time. The notation of (STA11) or the like within the frame represents a destination wireless communication station. For example, (STA11) represents that the destination is the wireless station STA11. In addition, the NAV (RTS) represents that the NAV is set after reception of an RTS which is not addressed to the station itself. Here, an example in which the wireless access point AP1 and the wireless stations STA11 to STA13 are present as the wireless communication stations and the wireless access point AP1 accommodates data for the wireless stations STA11 to STA13 and transmits frames addressed to the wireless stations STA11 to STA13 is shown. The wireless access point AP1 acquires the TXOP and transmits data on an 80 MHz channel to the wireless station STA13, which can use a largest band among the destination stations. When data communication with the wireless station STA13 ends, the wireless access point AP1 transmits data to the wireless station STA12, which can use a second largest band among the destination stations, and finally transmits data to the wireless station STA11, which can use a smallest band among the destination stations.
Hereinafter, operations of the wireless access point AP1 and the wireless stations STA11 to STA13 will be described with reference to FIG. 73. First, the wireless access point AP1 executes CSMA/CA when data for the wireless stations STA11 to STA13 is generated, and acquires the transmission right (TXOP) after confirming that a signal transmitted from another wireless communication station is not detected over a predetermined sensing period and a random back-off time. The wireless access point AP1 becomes the transmission right acquiring wireless communication station (TXOP holder) because the wireless access point AP1 has acquired the transmission right, and transmits a frame. The wireless access point AP1 transmits a request to send (RTS) frame serving as a start frame representing the start of the frame sequence for the wireless station STA13, which can use the largest band among the destination stations, to which data is to be transmitted (time t111).
Because the destination of the RTS frame received by the wireless station STA13 is the station itself and a transmission stop period is not set within the station itself, the wireless station STA13 replies a clear to send (CTS: transmission permission) frame for the wireless access point AP1 (time t112). Thereby, the wireless station STA13 notifies the wireless access point AP1 of the fact that the wireless station STA13 is in a state in which data can be received.
In contrast, because the destination of the RTS frame is not the wireless stations STA11 and STA12, which are the other wireless communication stations receiving the RTS frame from the wireless access point AP1, the wireless stations STA11 and STA12 set a period represented by the continuous use period information included in the RTS frame as an NAV period (transmission stop period) and do not perform frame transmission within the NAV period. In addition, the wireless stations STA11 to STA13 detect that the TXOP period (use transmission right period) starts because the RTS frame is received from the wireless access point AP1 and store the fact that the wireless access point AP1 is the transmission right acquiring wireless communication station (TXOP holder).
Subsequently, when the CTS frame is received from the wireless station STA13, the wireless access point AP1 transmits a frame for the wireless station STA13 (time t113). If the frame for the station itself is correctly received, the wireless station STA13 replies a block ACK (BA) frame (or positive acknowledgement (ACK) frame) for the wireless access point AP1 (time t114) and ends the transmission and reception of the frame.
Next, in order to transmit data for the wireless station STA12, which can use a second largest band among the destination stations, the wireless access point AP1 transmits an RTS frame, the destination of which is designated as the wireless station STA12 (time t115). Here, although the NAV is set in the station itself, the wireless station STA12 replies a CTS frame for the transmission right acquiring wireless communication station AP1 because the frame from the TXOP holder is received (time t116).
The wireless stations STA11 and STA13 set NAV periods because the RTS frame for another wireless station is received. In addition, if the NAV periods are already set, their NAV values are updated. If the CTS frame is correctly received from the wireless station STA12, the wireless access point AP1 transmits a frame for the wireless station STA12 (time t117). If the frame is correctly received from the wireless access point AP1, the wireless station STA12 replies a BA frame (or ACK frame) for the wireless access point AP1 (time t118) and ends the transmission and reception of the frame.
Next, in order to transmit data for the wireless station STA11, which can use a smallest band among the destination stations, the wireless access point AP1 transmits an RTS frame, the destination of which is designated as the wireless station STA11 (time t119). Because the RTS frame is received from the wireless access point AP1, which is the transmission right acquiring wireless communication station, the wireless station STA11 replies a CTS frame for the transmission right acquiring wireless communication station regardless of whether the period is within the NAV period (time t120).
In contrast, the wireless stations STA12 and STA13 set NAV periods because the RTS frame that is not addressed to the stations themselves is received. If the NAV periods are already set, the NAV values are updated. If the CTS frame is correctly received from the wireless station STA11, the wireless access point AP1 transmits a frame for the wireless station STA11 (time t121). If the frame is correctly received from the wireless access point AP1, the wireless station STA11 replies a BA frame (or ACK frame) for the wireless access point AP1 (time t122) and ends the transmission and reception of the frame.
Although the above description is an example of a frame sequence when a MAC protection technique by exchanging an RTS and a CTS is applied before the data is transmitted, a frame for data transmission may be transmitted immediately after an access right is acquired without exchanging an RTS and a CTS. In addition, the above description is an example in which a frame is transmitted for a plurality of stations within the same TXOP section. It is possible to transmit a frame to a plurality of stations as described above in a range which does not exceed an upper limit of the TXOP prescribed in the IEEE 802.11 standard. In addition, in this case, it is impossible to perform communication using a larger channel width than a channel width used once within the TXOP period. That is, although it is impossible to widen a width of a channel to be used in the TXOP section, it is possible to narrow the channel width if necessary. In the case of the example of FIG. 73, frames are transmitted in order from a destination in which an available channel width is larger because the wireless station STA11 can use channel 1 (CH1), the wireless station STA12 can use CH1 and CH2, and the wireless station STA13 can use CH1 to CH4.
Next, channel bandwidths used at the time of data transmissions between the wireless access point AP1 and the wireless stations STA11 to STA13 will be described with reference to FIG. 74. FIG. 74 is a diagram illustrating the channel bandwidths used at the time of the data transmissions between the wireless access point AP1 and the wireless stations STA11 to STA13. Because the wireless station STA11 can use only 20 MHz, the wireless access point AP1 communicates with the wireless station STA11 via channel 1 (CH1).
In Non-Patent Document 3, a unit channel to be necessarily used regardless of a transmission bandwidth when communication is performed within a cell constituted of a certain access point and stations is defined and it is called a primary channel. On the other hand, a channel which is used when communication is performed but is not the primary channel is called a secondary channel or a secondary x MHz channel (where x is a number among 20, 40, and 80) in Non-Patent Document 2. In the present Description, among all bands used in the cell, an arbitrary unit channel which is not the primary channel is referred to as a secondary channel.
FIG. 75 illustrates an example of the primary channel and the secondary channels when the unit channel is 20 MHz and the entire band used in the cell is 80 MHz. In FIG. 75, an example in which there are three secondary channels is illustrated.
Because the wireless station STA12 can handle up to 40 MHz, communication between the wireless access point AP1 and the wireless station STA12 is performed on the primary channel of 20 MHz and 20 MHz (secondary channel) adjacent to the primary channel (that is, on CH1 and CH2). In addition, because the wireless station STA13 can handle up to 80 MHz, communication between the wireless access point AP1 and the wireless station STA13 is performed on the primary channel and three secondary channels.